There is a growing trend of designing precision band-gap voltage reference circuits that are suitable for production test. Such circuits are always an essential component in massive digital, analog and mixed-signal circuits, as they provide precise reference voltages and/or reference currents, which have low sensitivity to power supply noise, temperature and process variations. In some cases, the reference currents may be inversely proportional to the value of a reference resistor.
Band-gap reference circuits provide reference voltages and/or reference currents. However, some band-gap circuits suffer from high temperature drifts and are unsuitable for high-precision applications. As a result, curvature trimming is an important and common post-fabrication step to achieve a low voltage drift over a range of temperatures, especially below 50 ppm/° C. In addition, a small start-up time is needed for efficient trimming during production test, because test time directly translates into chip cost. In contrast, large capacitors are typically needed to achieve low noise and high power supply rejection (PSR). However, such large capacitors would result in large start-up times, which are in conflict with the preferred requirements for test time. High-precision applications that require low output noise and high PSR from the band-gap reference output would require both small start-up times and large capacitors.
FIG. 1 shows a prior art band-gap voltage reference circuit (100) followed by a large resistor (101) and a large capacitor (102) forming an RC filter to reject high-frequency noises, thereby achieving low output noise and high PSR. As a result, the start-up time is large because the start-up time is proportional to the RC time constant.
FIG. 2 shows a prior art band-gap voltage reference circuit (200). The circuit consists of a conventional band-gap circuit made of two bipolar junction transistors Q1 (201) and Q2 (202), a resistor R1 (203) and two R2 resistors (204) and (205), an operational amplifier (206), two MOSFET current sources M1 (207) and M2 (208), and an output branch consisting of a MOSFET current source M3 (209) and a resistor R3 (210). The current flowing in the bipolar transistors Q1 (201) and Q2 (202) is proportional to absolute temperature (PTAT), while the current flowing in the R2 resistors (204) and (205) is complementary to absolute temperature (CTAT). As a result, the current in the MOSFET current sources M1 (207), M2 (208) and M3 (209) is almost independent of absolute temperature. However, due to the non-linear temperature dependence of the CTAT current that originates from the temperature dependence of the bipolar junction forward voltage, the reference voltage shows temperature dependence that is usually not acceptable in high-precision application.
To control the undesirable temperature dependence, the band-gap reference circuit (200) uses a bipolar junction transistor Q3 (211), two R4 resistors (212 and 213), and a MOSFET current source M4 (214) to subtract the non-linear temperature dependence of the CTAT currents, thereby yielding an almost constant reference with respect to temperature variations. In this design, there is no filter or capacitor to limit the bandwidth of the band-gap. Therefore, the start-up time will be small and suitable for production test. However, the PSR will be low and the noise will be high for this design.
Thus, the prior art band-gap reference circuit disclosed in FIG. 1 uses either a filter or large output capacitor to limit the bandwidth and get high PSR and low output noise. Such a band-gap reference circuit has a large start-up time and cannot be used in high-precision applications as it will require a large trimming time during production test. The band-gap reference circuit disclosed in FIG. 2 has a low sensitivity to temperature variations and a small start-up time suitable for production test. However, this prior art band-gap reference circuit has poor PSR and high output noise due to the lack of large capacitors and/or RC filters.
While these prior art band-gap reference circuits are useful, there is still a need in the semiconductor industry for band-gap reference circuits that satisfy the requirements for high precision applications (e.g. low temperature sensitivity, high PSR and low output noise) while also having a fast start-up time to support production test.